const num = 20

const list = [1,2,null]

window.onmousedown = function(mouseEvent) {
    console.log(mouseEvent.button);  //<- Error
};
interface Animal {

}
class Rhino implements Animal {

}
class Elephant implements Animal{}
class Snake implements Animal{}
function createZoo(): Animal[] {
    return [new Rhino(), new Elephant(), new Snake()];
}